Although sintered, porous expanded polytetrafluoroethylene (ePTFE) films and fabrics comprised of a laminate of such expanded, sintered, porous PTFE film and cloth have been proposed and used practically as waterproof, water-vapor-permeable films, they do not have adequate elastic properties.
For example, Japanese Unexamined Patent Publication No. 51-30277 (see U.S. Pat. No. 3,953,566) discloses a technology for obtaining an expanded, sintered, porous PTFE film having enhanced strength such as tensile strength as a result of expanding and increasing the porosity of a PTFE molded article, obtained by injection-molding a PTFE powder-containing paste, at a temperature equal to or below the crystal melting point of PTFE followed by sintering for 5 seconds to 1 hour at a temperature equal to or above the crystal melting point, such as a temperature of 350 to 370° C.
In addition, Japanese Unexamined Patent Publication No. 55-7483 contains a description regarding a material in which an elastomeric resin layer is formed on an expanded, sintered, porous PTFE film. Although these Japanese Unexamined Patent Publications Nos. 51-30277 and 55-7483 provide articles that can be preferably used as clothing, it is not the object of either of these publications to provide articles having elastic properties.
Japanese Unexamined Patent Publication No. 59-187845 discloses a method for imparting elastic properties by mechanically elongating a composite film of a sintered, expanded polytetrafluoroethylene (ePTFE) film and an elastomeric resin layer, or a composite in which that composite film is laminated with a cloth, beyond the yield point of the ePTFE at least by 5% or more. The ePTFE film of Japanese Unexamined Patent Publication No. 59-187845 is considered to be an expanded, sintered, porous PTFE film based on the description that it is given by the technology of U.S. Pat. No. 3,953,566.
The examples in this Japanese Unexamined Patent Publication No. 59-187845 describe that when a composite article of an ePTFE film and an elastomeric resin having a width of 12 inches (and length of 14 inches) is folded into a strip having a width of 1 to 1.25 inches and elongated two-fold (9-inch clamping interval elongated to 18 inches) in the longitudinal direction with an Instron tester, the sample width is necked to ⅜ to ½ inch. In this method, two-fold elongation treatment is possible by necking the sample during elongation treatment. Here, necking the width by about ½ during two-fold elongation means that the sample was only deformed in the direction of elongation without any substantial change in the size (total area) of the sample. In the case of having necked sample width during elongation, even if the tension during elongation is released, wrinkles form in the direction of elongation and the sample width only recovers to about 70 to 80% of the width prior to elongation. Consequently, in the case of a producing a product with this method, products can only be produced having a width that is narrower than the width of the cloth used in production, thereby increasing production cost. In addition, since wrinkles form in the product, the appearance becomes poor. In addition, in the case of having carried out elongation treatment in the longitudinal direction while preventing necking in the transverse direction under the conditions described above, due to the use of a sintered ePTFE film, breakage occurs due to elongation of roughly 20%, thereby making elongation treatment difficult.
Although elastic properties are not described in Japanese Unexamined Patent Publication No. 59-187845, it is described in Example 1 that the width is necked to about half at the stage of having elongated two-fold, and that stretch recovery immediately after is 64%. When the sample of Example 1 was actually prepared and subjected to an elongation percentage test according to the method described later, the sample width was necked to about half, the elongation percentage was 25%, and the elongation recovery rate was 65%, thus indicating that the sample cannot be said to have adequate elastic properties.
In addition, Japanese Unexamined Patent Publication No. 61-137739 discloses an elastic, water-vapor-permeable, waterproof film comprised of a composite film of an unsintered ePTFE film and an elastomeric resin. Namely, a method is disclosed for expressing elastic properties by impregnating and retaining an elastomeric resin having hydrophilic groups into an unsintered ePTFE film. Although the ePTFE film is unsintered, this is an essential requirement for expressing elastic properties. Since the ePTFE film is unsintered, a peel-off phenomenon of the surface layer occurs caused by insufficient cohesive force in the direction of thickness. It is described in this publication to the effect that, if sintering is carried out to avoid this, ductility decreases due to an absence of sliding between fibrils, and even if sintered ePTFE is laminated with other lamination material, the elastic properties of the partner lamination material, i.e. the other material, is inhibited by the sintered ePTFE, thereby causing the entire laminate to have hardly any elastic properties. In other words, it is suggested that simply impregnating and retaining an elastomeric resin in sintered ePTFE does not result in the expression of stretchability.
Moreover, Japanese Unexamined Patent Publication No. 61-137739 also describes the coating of a resin having hydrophilic groups on one or both sides for the purpose of avoiding the peel-off phenomenon of the surface layer. In the case of coating on one side, however, the lack of cohesive force on the uncoated side becomes pronounced, and even if both sides are coated, if voids remain within the ePTFE film, cohesive force is inadequate at those portions. In order to completely prevent the peel-off phenomenon of the surface layer, it is necessary to completely impregnate the inside of the ePTFE film with a resin having hydrophilic groups, which inevitably increases the thickness of the resin layer. Since resins having hydrophilic groups are hydrophilic, although water vapor permeability is expressed by dissolving moisture within the resin, as long as the moisture migrates by diffusing through a non-porous resin layer, the more water vapor permeability decreases, the greater the thickness of the non-porous resin gets. Although practical durability is inadequate unless unsintered ePTFE films are completely impregnated with a resin having hydrophilic groups to prevent the peel-off phenomenon of the surface layer and the like, complete impregnation makes it difficult to express a high level of water vapor permeability.